Laminated board for exterior cladding

ABSTRACT

Laminated board for exterior cladding formed from a core ( 13 ) of kraft paper impregnated with phenolic resin, at least one layer of natural wood ( 14 ) and at least one surface covering layer ( 15 ) including a coating film ( 17 ) and a substrate film ( 16 ). The coating film ( 17 ) is a single-layer film based on polymethyl methacrylate (PMMA), a two-layer film with its lower layer based on polymethyl methacrylate and its upper layer based on polyvinylidine fluoride (PVDF), or a PMMA/PVDF single-layer film. The substrate film ( 16 ) consists of a paper impregnated with a compound formed from 80-95% by weight of a phenolic resin and 5-20% by weight of a polymer promoting adhesion of the coating film ( 17 ), such as an acrylic, melamine or urea polymer or a combination thereof.

FIELD OF THE INVENTION

[0001] This invention relates to a laminated board for cladding surfacesexposed to the weather and, in particular, a laminated board having atleast one layer of natural wood.

PRIOR ART

[0002] The aesthetic properties of natural wood make it very desirableas a basic material for various types of construction components, andmore particularly exterior cladding components.

[0003] Various solutions have been put forward in the art to overcomethe problems deriving from the high cost of natural wood and itsdeterioration due to the weather. One of these comprises forming alaminated board by pressing a block comprising a base layer of acellulose material, (at least) one intermediate layer consisting of avery thin sheet of natural wood and (at least one) outer covering layerformed, from an appropriate material under specific pressure andtemperature conditions, the said materials being impregnated bysynthetic resins which help to create a very compact and strong boardthrough the application of pressure and heat.

[0004] In this respect Swiss patent no. 477272, published on the 15 Oct.1969, describes boards of this type having a base layer of cellulosematerial, two intermediate layers of wood sheet and two outer coveringlayers to provide adequate protection for the natural wood.

[0005] Of the various materials which have been used to form boards, anumber are known in the art.

[0006] Thus, with regard to the base layer, French patent no. 2096943,the application for which was published on the 3 Mar. 1972, describes inparticular the use of kraft paper impregnated with a phenol resin as abase layer or, using the terminology in that patent, as a reinforcingmember for the board. The material is being currently used for thispurpose.

[0007] As far as the covering layer is concerned, the art has continuedto offer a multitude of materials having useful properties to protectthe board in the very different environmental conditions to which aboard permanently exposed to the weather is subjected.

[0008] In this respect a large number of solutions specifically proposedfor boards which incorporate sheets of natural wood as solutions forvarious types of laminated boards of cellulose material formed bypressing various layers are known.

[0009] We will summarize some of these solutions briefly below.

[0010] French patent 2310872, the application for which was published onthe 10 Dec. 1976, mentions covering layers of cellulose paperimpregnated with melamine resin, which may also be coloured.

[0011] British patent application GB 2020199 A, published on the 14 Nov.1979, describes a process for the manufacture of a cellulose papercoated with melamine resin which can be applied to various types ofboards to impart resistance to abrasion and heat and the action ofparticular chemical products.

[0012] Spanish patent ES 8300047, published in 1983, describes two typesof covering layer: a resin-impregnated decorative layer which is stableto heat and an upper layer which provides a surface which proves to beprotective against abrasion formed for example by an alpha-cellulosepaper impregnated with a high quality heat-stable resin such asmelamine/formaldehyde, which is made transparent by consolidation of thelaminated assembly by heat and pressure.

[0013] European patent EP 0281586, the application for which waspublished on the 10 Mar. 1988, describes a laminated board with an upperlayer of acrylic resin.

[0014] More recently one or more skin materials, which, using theterminology conventionally used in the industry we will refer to belowas films, provided by the chemical industry to satisfy the coveringneeds of many different types of industrial products, have been used forthe surface covering layer of laminated boards.

[0015] Among these films, films based on polymethyl methacrylate (PMMA)are of a special interest for the upper part of the covering layer ofthe laminated boards which we are considering. However, the adhesion ofthese films to the films used in the lower part and in direct contactwith the sheet of natural wood such as, for example, films impregnatedwith phenolic resins, is a problem.

[0016] In application WO 01/05587 published on the 25 Jan. 2001 it isproposed that this disadvantage should be overcome by adding an adhesiveand an absorbent cellulose film between the acrylic film and the filmlocated on the sheet of natural wood, both films being impregnated witha resin of the same nature.

[0017] This solution is costly, and therefore this invention proposes analternative solution to the problem mentioned, with additionaladvantages.

SUMMARY OF THE INVENTION

[0018] The subject matter of this invention is a laminated board forexterior cladding which includes a core of kraft paper impregnated withphenolic resin, at least one sheet of natural wood and at least onesurface covering layer which includes a coating film providing the boardwith high resistance to various atmospheric agents (moisture, solarradiation, etc.) and a substrate film which provides good adhesionbetween the abovementioned coating film during manufacture of the boardby consolidation and compaction of its components through theapplication of pressure and heat.

[0019] Both a single-layer film based on polymethyl methacrylate (PMMA),which has good surface properties against various atmospheric agents,and a two-layer film on a lower layer based on polymethyl methacrylate(PMMA) and an upper layer based on polyvinylidine fluoride (PVDF), whichhas better performance than the above as a protective component for theboard, or a single-layer PMMA/PVDF film (with the PVDF dispersed in thePMMA) giving similar performance to the above, are also considered to beappropriate as coating films.

[0020] As a substrate film it is proposed that a paper impregnated witha compound formed from a phenolic resin in a proportion of between 80and 95% by weight and a polymer which promotes adhesion of the coatingfilm in a proportion of between 5 and 20% by weight should be used.

[0021] These adhesion-promoting polymers include acrylic, melamine andurea polymers and combinations of the same.

[0022] The fundamental advantage of this invention lies in thepossibility of using a number of high quality coating films provided bythe chemical industry for a variety of applications in the manufactureof laminated boards having a layer of natural wood, using theconventional pressing process under specific pressure and temperatureconditions and without the need to use specific layers of adhesive.

[0023] Other features and advantages of this invention will be apparentfrom the detailed description of the invention below, with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 shows a diagrammatical view of the transverse cross-sectionof a laminated board known in the art.

[0025]FIG. 2 shows a diagrammatical view in transverse cross-section ofa laminated board according to this invention.

DETAILED DESCRIPTION FO THE INVENTION

[0026] This invention is based on a board which the applicant hasmanufactured and marketed since the year 1994, having the trade namePARKLEX 1000, and which we will describe briefly with reference to FIG.1.

[0027] The basic components of board 1, all of which are available onthe market, are a core 3 of kraft paper impregnated with phenolicresins, thin sheets of wood 4, 4′ and covering layers 5, 5′ formed fromat least one film impregnated with a resin which imparts an aestheticappearance and appropriate surface properties for its purpose.

[0028] A person skilled in the art will understand that board 1 known inthe art may include components in addition to those mentioned, such asfilms having a decorative purpose in order, for example, to provide theboard with particular ornament or colour, or films placed between core 3and sheets of wood 4, 4′ to prevent the phenolic resins of core 3affecting the colour of the natural wood.

[0029] This assembly is subjected to specific pressure and heatconditions which bring about compaction and consolidation—in whichpolymerization of the resins mentioned has a determining role—yielding aproduct with the aesthetic appearance of natural wood but much superiorresistance to the weather.

[0030] According to this invention, the surface properties of the saidboard can be improved by modifying its covering layer in such a waythat, while the process of manufacture of the board by pressing itscomponents under specific pressure and temperature conditions isunchanged, some films offered by the industry, having properties whichare particularly appropriate for the boards which we are consideringsuch as, in particular, films based on polymethyl methacrylate (PMMA)and films based on polymethylmethacrylate (PMMA) and polyvinylidinefluoride (PVDF), can be used in the upper part of the covering layer.

[0031] An example of the former is PLEXIGLAS® 99845 film marketed byRöhm GmbH & Co. KG.

[0032] As an example of the latter mention may be made of EUROPLEX®99710 film marketed by Röhm GmbH & Co. KG and the KYNAR® PVDF filmsmarketed by Atofina Chemicals Inc. The following may be mentioned assignificant properties of these films:

[0033] mechanical strength and hardness

[0034] high resistance to abrasion

[0035] high thermal stability

[0036] high dielectric strength

[0037] high degree of purity

[0038] easy melting

[0039] resistance to most chemicals and solvents

[0040] resistance to ultraviolet rays and nuclear radiation

[0041] resistance to atmospheric agents

[0042] resistance to all types of fungi

[0043] low permeability for most gases and liquids

[0044] low levels of flame and smoke

[0045] As illustrated in FIG. 2, board 11 according to the invention hasa core 13 of kraft paper impregnated with phenolic resins and layers ofwood 14, 14′ precisely the same as board 1 previously known in the art.However, unlike it, covering layer 15, 15′ is formed from an outercoating film 17, 17′, such as those mentioned, and a substrate film 16,16′ which while providing a particular aesthetic appearance for theproduct also provides good adhesion for coating film 17, 17′.

[0046] It is intended in this way that the barrier properties of thecoating film against atmospheric agents such as UV radiation and waterwill be imparted to the board, thus appreciably improving its propertieswith respect to the said agents.

[0047] The applicant for this invention is not aware of any othercommercial product which can act like substrate film 16, 16′, aidingadhesion of coating film 17, 17′, without the need for a layer ofadhesive placed between the two films, but has found that this functioncan be satisfactorily performed by a thin sheet of paper impregnatedwith a resin formed from a phenolic resin as a major component and anadhesion-promoting polymer as a minor component (between 5 and 20% byweight).

[0048] Surprisingly it has been found that acrylic, melamine and ureapolymers, either pure or in combination, in proportions of between 5 and20% as mentioned, perform satisfactorily as polymers promoting theadhesion of single-layer coating films based on PMMA, such asPLEXIGLASS® 99845, or two-layer films with a lower layer based on PMMA,such as EUROPLEX® 99710.

[0049] Because of its solubility in water, the use of a melamine polymeris particularly useful in combination with water-based phenolic resins.In addition to this it has been observed that this polymer improvesresistance to UV rays.

[0050] The process for obtaining said substrate film 16, 16′ is theconventional one: passing a sheet of paper through a bath of resinhaving the composition mentioned and a set of impregnating rollers. Onleaving this bath the wet sheet passes through a drying tunnel in whichthe moisture level and the progress of the resin polymerization reactionare adjusted by controlling the temperature and the speed of the sheet.Although the above description has always mentioned a board having anasymmetrical structure, those skilled in the art will understand thatboards configured differently, especially on the side which is intendednot to be seen, will fall within the scope of the invention. In thesecases, on the side which is not seen, sheet of wood 14′ and coveringlayer 15′ may be replaced by other components. For example, instead ofacrylic or fluorinated coating films like those mentioned, other filmssuch as PVC, polypropylenes or polyesters which are not resistant to UVradiation but are resistant to moisture may be used.

[0051] Various examples of embodiments of the invention are describedbelow to provide a better illustration of the same.

EXAMPLE 1

[0052] A substrate film comprising a 42 g/m² paper impregnated with aphenolic resin (phenyl-formaldehyde resin of the resol type) in anaqueous base in which a melamine resin has been dissolved in aproportion of 10% (w/w) is placed on the outer surface of the woodenlayer of the board and on this is placed as a coating film EUROPLEX99.710 film from the Röhm company, with its acrylic part in contact withthe substrate film and leaving the PVDF film on the outside of theboard. This assembly is subjected to a pressure of between 5 and 10 MPaat 140° C. for 15 minutes.

EXAMPLE 2

[0053] A substrate film comprising a 42 g/m² paper impregnated with awater-based phenolic resin in which an acrylic melamine resin has beendissolved in a proportion of 10% (w/w) is placed on the outer surface ofthe wood layer of the board and on this PLEXIGLAS 99845 film from theRöhm company is placed as a coating film. This assembly is subjected toa pressure of between 5 and 10 MPa at 140° C. for 15 minutes.

[0054] We will show below the results of some tests performed withdifferent types of boards.

[0055] Firstly, we will show the results of adhesion tests in Table 1.The protective efficiency of the films against moisture and solarradiation depends on adequate adhesion to the surface of the board. Thisadhesion is evaluated by mere visual observation of the surfaceappearance of the board after it has been subjected to varioustreatments.

[0056] Table 1 summarizes the results of adhesion tests for:

[0057] board with EUROPLEX 99.710 film as the coating film placed on afilm of impregnated substrate with only one phenolic resin, (T2)

[0058] board according to example 1 (T3)

[0059] board according to example 2 (T4)

[0060] The treatments were:

[0061] no treatment (Ez)

[0062] immersion in water at 65° C. for 48 hours (E2)

[0063] immersion in boiling water for 2 hours (E3)

[0064] residence in a “Solarbox” ageing chamber for various periods oftime of between 100 and 1500 hours (E4100 to E41500). TABLE 1 T2 T3 T4E1 Very good Very good Very good E2 Satisfactory Very good Very good E3Poor Very good Good E4₁₀₀ Very good Very good Very good E4₅₀₀Satisfactory Very good Very good E4₁₅₀₀ Poor Very good Good

[0065] Very good: no signs of detachment. Unchanged

[0066] Good: very small blisters or very slight fogging

[0067] Satisfactory: small blisters or slight fogging

[0068] Poor: obvious blisters or fogging

[0069] Very poor: total detachment of the film.

[0070] These results show an obvious improvement in the adhesion of thefilms when substrate films according to the invention are used. Thisimprovement takes the form of a better surface appearance of the boardssubjected to immersion treatment in water.

[0071] Secondly, and in Table 2, we show the results of moistureabsorption tests in terms of the gain in weight of test specimensfollowing immersion in boiling water for 2 hours. Moisture absorption isa parameter of vital importance given that it is the combined effect ofwater, oxygen and solar radiation which initiates the main processes ofdegradation of board components. A board without a coating film (T1) hasbeen added to the boards tested. TABLE 2 T1 T2 T3 T4 E3 1.28% 1.17%0.66% 0.71%

[0072] These results show that the incorporation of coating filmsreduces water absorption and that this reduction is greater the betterthe level of adhesion with the substrate film. Two-layer films withfluorinated surfaces, which are therefore very hydrophobic, offer thebest performance.

[0073] Finally we show in Table 3 the results of accelerated ageingtests carried out in “Solarbox 1500” chambers and subjected to twodifferent “dry period/wet period” cycles (during the dry period the testspecimens are subjected to dry UV radiation and during the wet periodthey are subjected to the same radiation but are submerged in 1 mm ofwater). The cycles were 2 hours/2 hours, and 3 hours/1 hour.

[0074] Although the agressivity of the two cycles is different and thelevel of degradation is also different, the differences in behaviourbetween the different boards appear again, and the results of the twotypes of test are summarized in Table 3. In this case the evaluation wascarried out in accordance with ISO standard 105-A 02 (“Grey Scale”)which measures progressive bleaching of the surface from unchanged (5)to wholly bleached (1). TABLE 3 T1 T2 T3 T4 E4₁₀₀ 4 5 5 5 E4₅₀₀ 3-2 4* 55 E4₁₅₀₀ 1 3** 5 4

[0075] The results show that the incorporation of coating films whichact as filters for UV radiation and water appreciably improve theweather resistance of the boards and that this resistance is greater thebetter the level of adhesion to the substrate film. Two-layer acrylicfilms with fluorinated surfaces offer the best performance.

[0076] Although one embodiment of the invention has been described andillustrated, it is obvious that modifications within the scope of thesame may be made to it, and the invention should not be considered to berestricted to this embodiment, but only the content of the followingclaims:

1. Laminated board (11) for exterior cladding comprising a core (13) of kraft paper impregnated with phenolic resin, at least one layer of natural wood (14) and at least one surface covering layer (15) which includes a coating film (17) and a substrate film (16), characterized in that: a) the coating film (17) is a film with at least one layer of polymethylmethacrylate (PMMA) either pure or modified through the addition of other polymers, b) the substrate film (16) is a paper impregnated with a compound formed from a phenolic resin in a proportion of between 95 and 80% by weight and a polymer promoting adhesion of the said coating film (17) in a proportion of between 5 and 20% by weight.
 2. Laminated board (11) according to claim 1, characterized in that the adhesion-promoting polymer is an acrylic polymer.
 3. Laminated board (11) according to claim 1, characterized in that the adhesion-promoting polymer is a melamine polymer.
 4. Laminated board (11) according to claim 1, characterized in that the adhesion-promoting polymer is a urea polymer.
 5. Laminated board (11) according to claim 1, characterized in that the adhesion-promoting polymer is a combination of an acrylic polymer and/or a melamine polymer and/or a urea polymer.
 6. Laminated board (11) according to any one of claims 1 to 5, characterized in that the coating film (17) is a single-layer film based on polymethyl methacrylate (PMMA) modified with polyvinylidine fluoride (PVDF).
 7. Laminated board (11) according to any one of claims 1 to 5, characterized in that the coating film (17) is a two-layer film with an upper layer based on polyvinylidine fluoride (PVDF). 